To provide an image forming apparatus and a method for acquiring the thumbnails of a stored document, which can share authentication function for the thumbnail of the stored document, can distribute the load on a document management DB and can supply an external networking devices with the thumbnails of the stored document, without wasting resources.

The image forming apparatus 1, which supplies the thumbnails of the stored document, solves above task, by including: Web page service which acquires certification information, according to a thumbnail acquisition request from the networking device 100, acquires thumbnail storage location information on a thumbnail for display which has been replicated from the thumbnail of the stored document, and supplies the networking device with the information; and document management service which generates the certification information, transmits it to the Web page service 122, while the thumbnail storage location information on the thumbnail for display is read, according to the certification information and transmits the read information to the Web page service.